1. Field of the Invention
The present invention relates to a method of controlling a hot strip finishing mill for effecting continuous rolling by interposing loopers respectively between a plurality of consecutive stands and connecting a rear end of a preceding bar (a rolled material undergoing a rolling process at the present) to a top end of a next bar (a rolled material to be rolled next).
2. Related Background Art
A practice widely adopted by cold tandem rolling mills is a so-called flying thickness change wherein a pass schedule is changed without stopping the mill during a rolling process, and products having the same size or different sizes are continuously rolled. It is of importance in this flying thickness change to reduce an off-gauge length to the greatest possible degree and simultaneously prevent an occurrence of troubles such as a rupture of a strip by properly changing set values of a roll peripheral speed and a roll gap of each stand when a size change point passes through the mill.
A method of changing the above-mentioned roll gap and roll peripheral speed is disclosed as a method of controlling tandem mills in Japanese Patent Post-Exam Publication No. 55-11923. According to this method, there are predetermined roll peripheral speeds and roll gaps during a period for which the size change point passes through each stand and after the size change point has passed through the stand. These roll gaps and roll peripheral speed are each stored as set values. The roll gaps and the roll peripheral speeds are changed to these set values at predetermined timings by tracking the size change point.
On the other hand, a flying change technique in hot rolling mills appears on pp. 181.about.184 of, e.g., a collection of pre-manuscripts written by Hiroshi Kosuga, Kunio Sekiguchi and others, titled [Flying Gauge Change Control For Hot Strip Finishing Mill] in the 36th Plastic Working Association's Lecture Meeting, Oct. 6, 1985. According to this technique, the roll gap is changed by varying a reference thickness under a gauge meter AGC of each stand, and the roll peripheral speed is changed under optimum mass flow control.
The hot strip finishing mill includes rolling stands 1.about.7 disposed at predetermined spacings. Rolled materials 8 are each rolled to a target thickness on the delivery side of each stand. Further, mechanical loopers 9.about.14 are provided between the respective stands. The looper raises the rolled material 8 up to a certain height and, besides, gives a predetermined tension to the rolled material 8. The flying change in this hot strip finishing mill is defined as a technique of performing continuous rolling by connecting a rear end of a preceding bar to a top end of a next bar (a connecting point thereof is indicated by Q).
Herein at the connecting portion, as illustrated in FIG. 7, the preceding bar and the next bar on the entry side of a first stand are generally different in terms of a steel grade, a thickness H and a width W. Besides, the sizes are also different on the delivery side of a final seventh stand. Table 1 shows one example of pass schedule of the preceding bar and the next bar in that case.
TABLE 1 ______________________________________ F1 Ent. F1 F2 F3 F4 F5 F6 F7 ______________________________________ Preceding Bar (mm) 35.0 18.1 10.8 6.6 4.3 3.0 2.2 1.8 Next Bar (mm) 30.0 14.0 8.0 4.7 3.0 2.0 1.5 1.2 ______________________________________
In this example, it is required that the thickness on the delivery side of each stand be changed in front and in rear of the connecting point. Based on a changing method thereof, as shown in FIG. 8, the thickness is changed for a certain time (change time), with the connecting point being centered. This change time is determined by an upper limit of a change speed of set values of the roll gap and the roll peripheral speed or a limit to secure an operating stability. The change time is 0.5.about.2.0 seconds according to actual results obtained so far. Even within this change time, a mass flow balance between the mutual stands has to be kept, and the stable operation also has to be actualized.
Now, it is assumed that an interstand distance is set to 5 m, and an interstand rolled material speed is set to 10 m/s. If the change time is 1 second or more, a size change portion is located astride a plurality of stands, and it follows that the set values of the plurality of stands are simultaneously changed. Nonetheless, in a state where the sizes of the rolled materials vary momentarily on the entry and delivery sides of the plurality of stands, it is almost impossible to accurately estimate the set values of the roll peripheral speed and of the roll gap for keeping the mass flow balance.
Under such circumstances, the cold tandem rolling mills are constructed so that the size change potion is not located astride the plurality of stands by reducing the rolling speed when changing the size. According to the hot strip finishing mill wherein a temperature of the rolled material on the delivery side of the mill is required to be held at a target value, however, the situation is such that the rolling speed can not be reduced.
On the other hand, the flying thickness change control in the hot strip finishing mill can not be applied to such processing that the rolled materials different in terms of the steel grade, the thickness and the width are connected on the entry side of the mill, or continuous rolling is conducted with different strip sizes on the delivery side of the finishing mill.
Note that a thinkable method of connecting the preceding and next bars may involve the use of welding, press-fitting, engaging, etc.. However, a tensile or flexural strength at the connecting point is, it is considered, still smaller than at points other than the connecting point. As illustrated in FIG. 6, the loopers are disposed between the stands, and the rolled material is raised by this looper to produce a tension. In this case, there exists a possibility in which the flexure and the tension are given to the connecting point enough to rupture the connecting point.